1. Field of the Invention
The present invention relates in general to the field of monitoring techniques for controlling the processes for welding metals by means of laser, applicable for example to the process for welding pre-finished elements, such as moulded elements, sheets, elements obtained by melting.
More in particular, the present invention relates to a method for analyzing data generated by a spectroscopic sensor, that is an apparatus able to measure simultaneously the intensity of an optical radiation in several wavelength bands (spectrum).
In the present invention, this type of sensor is used to measure with said mode the optical emission (emission spectrum) of the plasma generated in the welding area. Such measurement is performed repeatedly during the phase of the welding process which ones wants to monitor.
The analysis method, by means of examining in real time the emission spectra acquired by said sensor, allows detecting the deviation of the physical parameters of the welding process from an optimum-considered region of values, deviation of parameters which, if it is not controlled and correct, can produce flaws in the resulting welding. The so-obtained piece of information can be used as entrance of a retroaction system of the welding apparatus, which intervenes in real time on the physical parameters of the welding process, in order to correct the anomaly. An integrating portion of this method for analyzing data is constituted by a calibration preliminary phase, wherein the measurements of said sensor are acquired corresponding to several welding processes performed both under optimized conditions, both under conditions wherein one or more parameters of the welding process are altered in a controlled way.
The spectra produced by the sensor during these tests are processed with the procedure better described hereinafter, in order to detect the correlations between the specifically introduced variation of the physical parameter and the features of the resulting emission spectrum. These sets of measurements constitute a library with which the measurements obtained under operating conditions will be compared, in order to detect deviations of the parameters from the optimum values thereof, which can take place during the welding process.
2. Description of the Prior Art
The assembly by means of laser welding of half-finished metallic elements is widely used in several industrial fields. In particular, in the field of the automobile production, the laser welding is used to join half-finished portions, obtained for example by moulding, which compose frame members or other chassis members (for example doors).
Among the most widespread laser technologies, currently there are the continuous laser welding implemented with carbon dioxide laser (CO2, emission at the wavelength of 10.6 micrometers) or solid state laser with Neodymium crystal or glass (Nd:YAG or Nd:Glass, with emission around 1.06 micrometers) with continuous operation, and the spot welding implemented with laser with pulsed operation.
As far as the continuous laser welding is concerned, such technique is industrially used by several producers by means of automatic or robotized systems, thus without the operator direct intervention.
The quality control aimed at reducing flaws in such welding processes constitutes a problem of considerable importance, particularly relating to the vehicle portions with passive safety function (for examples structures with controlled deformability in the chassis front portion) wherein welding flaws, which might compromise the resistance thereof in case of collision, are not admitted.
The quality control often is post-performed, that is at the end of the production process, with methods such as the visual inspection or other not destructive diagnostic methods (for example radiographic methods, ultrasound survey) or destructive methods (resistance tests, welding junction sections) on a limited number of samples. This quality control phase is very time-consuming, with consequent impact on the production costs.
Furthermore, in case flaws are found, it is not always possible obtaining indications about the causes which have brought about the flaw, or the remedies to be adopted to eliminate the flaw itself.
Another approach to the quality control in the laser welding is constituted by the line control, during the welding process, by means of proper sensors, for example of optical or thermal type.
With these sensor techniques it is possible monitoring the welding process and detect the deviation of the process conditions with respect to the optimum values, in order to detect and possibly correct in real time the flaw occurrence.
In order to explain the objects and the features of the present invention, it is better to provide a brief description of the process for the continuous laser welding of metallic sheets, as well as the process for forming plasma and for luminous emission generated by such plasma.
The autogenous welding process is obtained by means of the localized melting of the metal constituting the members to be joined. In case of continuous laser welding, the localized melting is obtained by focalizing a laser beam with adequate intensity on the metal surface.
The absorption by the metal of a laser beam power fraction determines an increase in the material temperature which reaches first of all the melting point and then the vaporization point of the metal. The so-generated vapour pressure expels the metal melt by the focal area along the beam axis, by generating a thin channel, said keyhole.
The keyhole opening allows the intensity supplied by the laser beams to deeply penetrate in the metallic piece, until reaching the underneath piece to be joined by means of the welding process.
The vaporized metal is further heated by the incident laser beam, until reaching a plasma state at high temperature, wherein there is the coexistence of:                atoms of the metal and gas of the surrounding atmosphere, under electrically neutral conditions, but under excited energy states;        atoms of the metal and gas of the surrounding atmosphere, under ionized conditions, and in excited energy states of ionized atom;        free electrons.        
Due to the high density of free electrons, the so-generated plasma results to be optically opaque in the region of the infrared wavelength, and only partially transmitting in the spectral region of visible and near infrared. Such property influences the power transferring process from the laser beam to the material to be welded: when the plasma is developed, the laser beam power absorption does not take place at the level of the metal surface, but due to the plasma, which in turn transfers heat to the surrounding metal (then one speaks of plasma-mediated process).
This effect is important in particular when the welding is performed with laser with emission in the spectral region of the infrared medium, such as for example the CO2 laser.
The radiative de-exciting from excited energy levels towards levels of lower energy of the atoms and ions composing the plasma causes the optical radiation emission at specific wavelengths depending from the features of the atom (or the ion) which de-excites and from the pair of energy levels involved in the transition. The radiation intensity I(λ) is determined by the atomic transitions taking place in the metal, but even by outer factors such as in particular:                plasma local temperature (line broadening due to the Doppler effect);        local pressure (line broadening due to collisions);        electronic density (broadening due to Stark effect),furthermore, the presence of free electrons determines the shifting (again due to Stark effect) of the wavelengths of the involved energy transitions.        
The whole plasma emission is constituted by the overlapping of the emissions generated by various ionic atomic species, each one thereof contributes with the emission linked to various transitions; furthermore the intensity and shape of each emission line depends upon the plasma local conditions (temperature, density, pressure, electronic density).
An example of the course of the plasma emission whole intensity in terms of the wavelength (hereinafter defined emission spectrum) is shown in FIG. 1.
It results then that the emission spectrum whole course is influenced by several elements concurring to the welding process, for example:                involved chemical species, belonging to the processed metals (in the mass or in possible coatings), shielding gases, possible contaminants, possible intrusion of environment air;        plasma temperature, ionization level of the involved species; and        electronic density in the plasma.        
These elements and physical quantities influence the process of transferring power from laser beam and the plasma and from the plasma to the pieces to be welded. Furthermore, they are influenced by other factors such as variations in the composition or in the thickness of the materials and variations in the process geometry, for example presence of gaps among the materials to be welded.
Although basically it is possible determining these physical quantities by means of a suitable analysis of the emission spectrum, with the current knowledge it is not possible establishing in advance a link between the plasma features and the welding quality, due to the complexity of the involved interaction processes.
Hereinafter a survey method of phenomenological type is described, therewith in the emission spectra differences are detected which are significant for detecting flaws, that is the cases wherein the welding is implemented in optimum way and the cases wherein the welding is performed under conditions producing a flaw.
As to the diagnostics performed by means of the acquisition of the plasma emission spectrum with a sensor of spectroscopic type, hereinafter some documents belonging to the here discussed state of art are evidenced.
German patent Nr. DE 4313287 described a method for analyzing the spectrum based upon the calculation of the intensity ratio between ion emission lines of a same element at different ionization states, in order to obtain information about the plasma ionization state, to be correlated to the welding penetration degree. Furthermore, in such method the ionization state provides indications about the result of the welding process as a whole, without specific indications about possible alterations of the parameters.
European patent Nr. 911,109 describes the use of a spectroscopic survey method aimed at detecting the plasma-emitting spectral bands useful for monitoring; such information is used to select one or more pass-band filters coupled to optical sensors; the process monitoring is performed by measuring the intensity of the light signal in the bands determined by the pass-band filters, and by verifying that in the process such intensity keeps within determined thresholds; such thresholds are established based upon the values which the signal assumes when weldings are performed with a result considered to be acceptable.
Furthermore, in this document it is claimed that the temporally mediated value of the signal coming from the above-mentioned sensors is correlated to one or more of the following parameters: translation speed of the piece to be welded; laser power; laser focus position; object surface contamination; shielding gas flow; object physical deformation.
U.S. Pat. No. 7,129,438 described instead the use of a methodology wherein the output signal of a general (ex. optical, image or acoustic) monitoring sensor is created to monitor a process parameter, and the output of such sensor (in case processed with a general mathematic algorithm able to provide a univocal result) is correlated to the welding quality by means of direct comparison with the weldings obtained by varying a single parameter of the welding itself.